Also performs duplicate detection for referenced aliases. Duplicate mappings might occur
when the referenced alias is referencing several mappings (in case of the referenced class
actually contructing an object tree). Mappings that exist in the base class will be travesrsed
twice without the duplicate detection. The onBeginMultipleMapping(ClassMapping,org.compass.core.mapping.Mapping)
detects such mappings, processes only the first one, and returns false for the rest
(denoting not to continue the investigation of this referenced mapping).